1. Field of the Invention
This invention relates to data processing apparatus and more particularly to data processing apparatus having fixed read only memories. The invention especially relates to apparatus for providing substitutions and corrections for errors or oversights in such fixed read only memories.
2. Description of the Prior Art
With the advancement of large scale integrated circuit technology toward providing faster, more efficient and less costly integrated circuit data processing memory elements, the data processing technology has been advancing in the direction of using fixed read only memory structures formed from large scale integrated circuits in place of the earlier and traditional units of random access memory which could be programmed and reprogrammed as desired to effect changes. In contrast, the read only memories (ROMS) are hard wired, i.e., preprogrammed in advance and cannot be changed. This, of course, has led to some difficulties in the data processing technology since errors can occur in the design and preprogramming of such ROMS, and operational needs cannot always be conceived in advance. This requires changes in the ROM memory to be made even in the field.
When the need for substitution or change arises, the costly approach would be to replace the undesired ROM memory structures with redesigned or changed structures. Such approaches require remanufacture. They can be relatively costly, particularly when the replacement has to be made on a considerable number of data processors. Also, they are slow because the replacements often take months to redesign and remanufacture. Accordingly, the technology has moved in a direction of field alterable units such as that described in U.S. Pat. No. 3,934,227 in which a mapping memory is provided which is connected to both the overlay memory and a fixed memory. The mapping memory is addressed simultaneously with the fixed ROM memory and the overlay memory. When so addressed, the mapping memory makes the decision as to whether a fixed memory unit should be used or a unit from the overlay memory substituted therefor. These mapping memories are preprogrammed to predetermine these decisions.
While such overlay memory units provide a solution to the problem of patching corrections into fixed ROMS in the field, they are generally limited in their flexibility and provide essentially for "one for one" substitution. They are also limited in their flexibility. In addition, once a substitution or patch is put in, it can never be undone again.